Power transmission system

ABSTRACT

A power transmission system includes a rotatable and drivable first shaft ( 1   a ) adapted for co-action with a motor ( 1   c ) included in a planetary gear arrangement (P), and further includes a second shaft ( 8 ) that can be driven by the planetary gear arrangement. The planetary gear arrangement ( 1 ) includes a centrally positioned sun gear ( 1   b ) that co-acts with said first shaft, a plurality of planet gears ( 1   d ) that can co-act with the sun gear, and a ring gear ( 1   e ) that co-acts with the planet gears, and a gear or wheel ( 1   f ) that holds the planet gears. A hydraulic system ( 10 ) belonging to the planetary gear arrangement (P) includes a pump unit ( 10   a ) and a motor unit ( 10   b ) among other things. The ring gear ( 1   e ) can be braked via a first coupling unit ( 3 ), so as to be stationary in relation to a chassis ( 4 ) in a low speed range, but free for rotation in a higher speed range. The first shaft ( 1   a ) functions to cause the sun gear ( 1   b ) to rotate. The hydraulic pump unit ( 10   a ) is coupled directly or indirectly to the gear ( 1   f ) that holds the planet gears. The hydraulic motor unit ( 10   b ) is coupled directly or indirectly to the second shaft ( 8 ). In a chosen high speed range, a second coupling unit ( 5 ) functions to co-ordinate the rotary movement of the ring gear ( 1   e ) with the rotary movement of the hydraulic motor ( 10   b ).

FIELD OF INVENTION

The present invention relates to a power transmission system thatincludes a rotatable and drivable first shaft which is adapted forco-action with a motor that forms part of a planetary gear arrangement,and a second shaft which is driven by said arrangement, wherein a planetgear belonging to the planet gear arrangement includes a centrallypositioned sun wheel that co-acts with said first shaft, a number ofplanet wheels co-actable with said sun wheel, and a ring wheel co-actingwith the planet wheels, and wherein an hydraulic system belonging to theplanet wheel arrangement includes a pump unit and a motor unit, amongother things.

The inventive power transmission system is adapted to drive a vehicle,preferably a cross-country vehicle, such that the first shaft can beconveniently-driven at a constant or gene. rally constant speed, so astherewith to drive an engine at a speed that corresponds to lowcontaminant emissions, high fuel economy and/or maximum power,regardless of vehicle driving conditions.

A power transmission system of the aforesaid kind is used to advantagein engine-powered vehicles where there is a need to be able to drive thevehicle in a low speed range, such as cross-country driving, or in ahigh speed range, such as along roads and highways.

More specifically, the present invention relates to a power transmissionsystem that is based on the use of a planet gear in which the ring wheelbelonging to said planet gear is braked and stationary in relation tothe chassis when the second shaft, the output shaft, is drivenhydraulically within a low speed range and where the ring wheel isfree-running and can rotate within a high speed range.

In the following description, the low speed range is illustrated as 0-25km/hr and the high speed range as 0-50 km/hr, although these speedranges shall only be seen as examples of speed ranges.

The following description describes the co-action between differentwheels, by which is meant preferably nonslipping co-action, such as agearwheel co-action.

By direct coupling is meant direct co-action between two or more wheels,and by indirect coupling is meant co-action caused through the medium ofone or more co-actable wheels.

By “wheel” is primarily meant a “gearwheel” in tooth co-action with oneor more adjacent gearwheels.

BACKGROUND OF THE INVENTION

Several different kinds of such-power transmission systems are known inthe art.

An example of such systems is described in Patent Publication DE-3 303886. This publication describes and illustrates a planetary geararrangement which includes a sun wheel (13) connected to a drive shaft(12), at least two planet wheels (18, 19), and a ring wheel (20) whichfunctions to drive an output shaft (22) while using an hydraulic torqueconverter (27, 28, 29).

The torque converter comprises two hydraulic pumps (27, 28); eachco-acting with its individual planet wheel (18, 19), and connectedhydraulically via a hydraulic conduit system (29).

Patent Publication U.S. Pat. No. 3,866,490 describes and illustrates atransmission system which uses a planet gear arrangement to regulate thespeed of an output shaft in relation to an input shaft.

More specifically, the input shaft (11) is connected directly fordriving the planet wheel carrier (16) via a connection sleeve (17), andtherewith the planet wheels (18, 19).

The outputs of this planetary gear assembly (15) comprise the ring wheel(20) on the one hand and the sun wheel (21) on the other hand.

A shaft (29) co-acts with a gearwheel (30) which, in turn, co-acts witha gearwheel (31) and drives an hydraulic unit (33) with variabledisplacement.

Patent Publication U.S. Pat. No. 4,750,381 teaches an hydromechanicaltransmission that can be used as an efficient, continuously variabletransmission with the aid of a differential mechanism.

This publication discloses the use of an hydraulic transmissionmechanism, a mode selecting mechanism, a locking mechanism and a clutchmechanism.

It will be noted in particular that the input shaft (11) acts on theplanet wheels, and that the sun wheel is coupled directly to a pump unit(5) having a fixed pump/motor displacement, which is coupledhydraulically to a variable displacement unit (7).

The publication also describes the use of a locking mechanism (11)related to the sun wheel shaft and a clutch (12) belonging to the unit(7).

Patent Publication GB-A 1 174 778 describes and illustrates atransmission mechanism which is particularly adapted for automotivevehicles, said transmission mechanism being of the kind in which poweris divided between a mechanical path and an hydrostatic path.

As with the present invention, this transmission mechanism is adaptedparticularly for use with cross-country vehicles, such as tractors, andcarries out a major part of its work under conditions that require a lowspeed range and a high torque.

Patent Publication GB-A 1 379 213 illustrates and describes anhydromechanical transmission system that is able to offer a plurality ofdriving speed ranges.

There is used to this end an input shaft (10) and an output shaft (12),and a duplicated planetary gear arrangement (4) and an hydrostatictransmission (16) that can co-act with the two planetary geararrangements (14).

SUMMARY OF THE INVENTION Technical Problems

When taking into consideration the technical deliberations that a personskilled in this particular art must make in order to provide a solutionto one or more technical problems that he/she encounters, it will beseen that on the one hand it is necessary initially to realise themeasures and/or the sequence of measures that must be undertaken to thisend, and on the other hand to realise which means is/are required insolving one or more of said problems. On this basis, it will be evidentthat the technical problems listed below are highly relevant to thedevelopment of the present invention.

When considering the prior art as described above, it will be seen thata technical problem resides in creating in a power transmission systemof the aforedescribed nature conditions which will enable the system tooperate within both a low speed range and a high speed range, and torealise the significance of allowing the ring wheel to be braked andmade stationary relative to the chassis with regard to the low speedrange and to allow the ring wheel to rotate in the high speed range, andtherewith realise the significance of allowing the input shaft to drivethe sun wheel and allowing the planet wheels to drive an hydraulic pumpunit which is coupled hydraulically to an hydraulic motor, said unitfunctioning to drive the output shaft.

Another technical problem is one of realising the significance ofcreating while using one single planetary gear assembly including sunwheel, planet wheel and ring wheel such that the low speed range solelyutilises an hydraulic system, a variable displacement hydraulic motor,and-an hydraulic clutch arrangement for speed control while using themechanical planet wheel function in the high speed range, and by causingthe hydraulic system to provide a split torque function in this latterapplication.

In respect of a power transmission system of the aforedescribed nature,a technical problem resides in realising the significance of adapting afirst coupling or clutch unit for action between the ring wheel and/or afirst gearwheel co-acting with said ring wheel, and a chassis, andadapting a second coupling or clutch unit for action between a secondgearwheel co-acting with said hydraulic motor unit and said firstgearwheel so as to drive said gearwheels simultaneously.

Another technical problem is one of realising the significance ofcausing the first coupling element to be in an actively coupled positionor state and to lock the ring wheel against rotation in the first speedrange, while causing the second coupling unit to be in aninactively-coupled state and therewith permit relative movement betweensaid two gearwheels or the like, where said second gearwheel isrotatably mounted on the second shaft, the output shaft, and in that insaid high speed range the first coupling element is in an inactivelycoupled state and releases the ring wheel for rotation, whereas thesecond coupling unit is in an actively coupled state and co-ordinatesthe rotary movement of the two gearwheels or the like.

Another technical problem resides in creating in a power transmissionsystem of the aforedescribed nature conditions whereby the first shaftis adapted to drive the pump unit either directly or indirectly in thelow speed range, through the medium of a wheel or drive coupled to theplanet wheels, said drive functioning as a planet wheel carrier.

Another technical problem is one of realising the significance ofcoupling the hydraulic variable displacement pump unit drivingly to themotor unit, said motor unit also being adapted for displacement controland said coupling being effected via an adapted hydraulic valvearrangement.

Another technical problem is one of realising the significance ofrotatably arranging the motor unit for adjustment of the motor power viadisplacement control, and of rotatably arranging the pump unit foradjustment of the pump power via displacement control, and therewithalso realising the significance of using a pump unit (or a motor unit)that can be regulated within the positive and/or the negative range.

Still another technical problem is one of realising the significance ofand the advantages associated with allowing the first and/or the secondcoupling unit to be activated and deactivated through the medium of avalve arrangement belonging to the hydraulic system.

Another technical problem is one of realising the significance of andthe advantages associated with enabling the maximum displacement of thepump unit to be equal to half, or essentially half, the maximumdisplacement of the motor unit.

It will also be seen that a technical problem also resides in theprovision of conditions with the aid of simple means and measures thatwill enable a low hydraulic flow to be regulated and controlled whileeliminating the risk of oscillation and run-away effects.

Solution

With the intention of solving one or more of the aforesaid technicalproblems, the present invention takes as its starting point a powertransmission system that includes a rotatable and drivable first shaftwhich is adapted for co-action with a motor forming part of a planetarygear arrangement, a second shaft which is driven by said planetary geararrangement, wherein a planet gear belonging to said planetary geararrangement includes a centrally placed sun wheel which co-acts withsaid first shaft, a number of planet wheels co-actable with said sunwheel, a ring wheel that co-acts with said planet wheels, a planet wheelcarrier, and an hydraulic system which belongs to the planet geararrangement and which includes a pump unit and a motor unit, and whereinthe ring wheel belonging to said planet gear can be braked via a firstcoupling unit and therewith remain stationary relative to a chassis in alow speed range or released for rotation in a high speed range, andwherein the hydraulic system is adapted to drive the second shaft withpump units and motor units in the low speed range, while providing asplit-torque function in the high speed range.

In accordance with the invention, a power transmission system of thiskind is characterised in that the first shaft is adapted to rotate thesun wheel, in that the hydraulic pump unit is coupled either directly orindirectly to the planet wheel carrier, in that the hydraulic motor unitis coupled to the second shaft either directly or indirectly, and inthat the system includes a second coupling element which functions toco-ordinate the rotary movement of the ring wheel with the rotarymovement of the hydraulic motor in the high speed range.

As proposed embodiments that lie within the scope of the inventiveconcept, it is also proposed that a first coupling unit shall be adaptedto act between a wheel co-acting with the ring wheel and the chassis,and that the second coupling unit shall be adapted to act between awheel co-acting with said motor unit and the wheel co-acting with saidring wheel for common rotary movement and rotation of the second shaft.

It is also proposed that said first coupling unit is in an activelycoupled state in said low speed range, while said second coupling unitis in an inactively coupled state, and that said first coupling unit isin an inactively coupled state in said high speed range while the secondcoupling unit is in an actively coupled state.

It is also proposed that a speed adjustment can be effected in the lowspeed range solely through the medium of an hydrostatic process betweenan hydraulic pump unit and an hydraulic motor unit having variabledisplacement, while a speed adjustment is made in the high speed rangewhile using the planet gear function, one with a rotatable ring wheel,and said hydraulic system.

It is also proposed in accordance with the invention that the firstshaft is adapted to drive said pump unit indirectly in said lowspeed,range, via the planet wheel carrier and a wheel connected to saidcarrier.

According to another embodiment of the invention, the pump unit has avariable displacement and is coupled to drive said motor unit, whichalso has a variable displacement, via an hydraulic valve arrangementthat has its own power source.

The invention is also based on the realisation that when the secondshaft rotates at a high speed within the high speed range, the motorunit present in the hydraulic system will be set essentially to zero andtherewith freely rotatable, and that the pump unit will be preventedfrom rotating in this motor setting and is essentially stationary,wherein the rotational movement of the first shaft is transferreddirectly to the second shaft via the sun wheel, the planet wheel and thering wheel.

According to another embodiment, the hydraulic motor unit is pivotallymounted for adjustment of the motor power through the medium of changingthe displacement, and the hydraulic pump unit is pivotally mountedarranged for adjustment of the pump power via a displacement adjustmentwithin the positive and/or the negative range.

According to another embodiment, the first and/or the second couplingunit is/are activatable and de-activatable through the medium of a valvearrangement belonging to an hydraulic system.

It is also proposed in accordance with the invention that the maximumdisplacement of the pump unit will be equal to half, or essentiallyhalf, the maximum displacement of the motor unit.

Advantages

Those advantages primarily afforded by an inventive power transmissionsystem reside in the provision of conditions whereby the speed of avehicle can be readily varied within a low speed range with low lossesand while utilising a planet gear and an hydraulic system, by solelyutilising the hydraulic system and blocking the effect of the ring wheelof said planet gear, while utilising the planet gear function in ahigher speed range with the aid of said hydraulic system, and therewithinfluencing the speed range by providing a split torque function.

The primary characteristic features of an inventive power transmissionsystem are set forth in the characterising clause of the following claim1.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to anembodiment at present preferred and having features significant of thepresent invention, and with reference to the accompanying drawings, inwhich

FIG. 1 illustrates a planetary gear assembly from above;

FIG. 2 is a sectional view of the planetary gear assembly shown in FIG.1, supplemented with necessary clutch or coupling units;

FIG. 3 illustrates the principle of speed control in a first, low speedrange;

FIG. 4 illustrates the principle of speed control in a second, higherspeed range;

FIG. 5 is a perspective view of an inventive planetary gear arrangement,but with the hydraulic clutch arrangement not shown;

FIG. 6 illustrates part of an hydraulic coupling system which actuatestwo separate coupling units;

FIG. 7 illustrates the power transmission system in a low speed mode, acreep-speed mode, and shows a first embodiment of a couplingarrangement;

FIG. 8 illustrates the system shown in FIG. 7 at a speed of 12 km/hr,and also shows a second embodiment of a coupling arrangement;

FIG. 9 shows the system of FIG. 8 at a speed of 25 km/hr;

FIG. 10 shows the system at the higher speed range, a speed of about 40km/hr;

FIG. 11 shows the system of FIG. 10 at a speed of about 50 km/hr;

FIG. 12 shows the system coupled for an overdrive function; and

FIG. 13 is a perspective view of the planetary gear arrangement havingdeliver lines, etc., for the hydraulic coupling arrangement of the firstembodiment according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS AT PRESENT PREFERRED

FIG. 1 is a side view of a known planetary gear mechanism 1 whichincludes a rotatable and drivable first shaft 1 a adapted for co-actionwith a motor (1 c) not shown in FIG. 1 and also for co-action with a sungear 1 b.

The sun gear 1 b co-acts (via tooth engagement) with three symmetricallyorientated planet gears, of which one is referenced 1 d. The planetgears 1 d co-act (via tooth engagement) with a ring gear 1 e. The planetgears 1 d are held firmly by a gear or drive if functioning as a planetgear carrier via their respective shafts, such as the shaft 1 d′.

The invention will be described with the assumption that the first shaft1 a is driven at a constant speed and that the speed at which the outputshaft 8 is rotated can be varied within predetermined values (0-25 km/hror 0-50 km/hr).

It is also presumed that the teeth of respective gears co-act with oneanother, although some other equivalent co-action can be envisaged.

As will be seen from FIG. 2, the shaft 1 a is fixed in relation to thesun gear 1 b, and the gear or drive 1 f is rotatably connected to theshaft 1 a via a bearing arrangement, therewith enabling the gear ordrive 1 f to rotate in a manner that deviates from the rotationalmovement of the shaft 1 a.

As will also be apparent from FIG. 2, the ring gear 1 e co-actsperipherally with a gear or drive 2, a so-called split-torque drive,said drive 2 being mounted on bearings for rotation relative to a shaft1 g.

A first coupling unit 3 is adapted to lock the gear or drive 2 inrelation to a stand or chassis 4 in an activated state of said unit 3,therewith preventing the ring gear 1 e from rotating.

The ring gear 1 e may alternatively be locked with the aid of othermeans or measures.

When the coupling unit or clutch 3 is in an inactive state, rotarymovement of the ring gear 1 e can be transmitted to the shaft 1 g viathe drive 2 and also via a second coupling unit or clutch 5, in a mannerdescribed in more detail hereinafter.

The coupling unit 3 has the form of a disc clutch and when compressed(active state) latches the gear or drive 2 against the chassis 4, andthus holds the gear stationary.

The gear 2 latches the ring gear 1 e, wherewith rotary movement of theshaft 1 a is transmitted directly to the planet gears 1 d whichtherewith rotate the planet carrier or holder 1 f in peripheral toothengagement with a gear 9.

The coupling unit 5 has the form of a disc clutch and when compressed(activated) brings the gears 6 and 2 to a position or their commonrotation. Since the gear 6 is fixedly connected to the shaft 1 g, thecommon rotation of the gears 2, 6 will drive the shaft 1 g and therewiththe gear 7 fixedly connected to the second shaft 8.

Driving of the shaft 1 g also drives the hydraulic motor, or vice versa.

The inventive planetary gear arrangement “P” shown in FIG. 5 includesthe aforedescribed planetary gear mechanism 1 and also an associatedhydraulic system 10 of a kind that will be described in more detailhereinafter with reference to FIGS. 7 and 13 and FIG. 8 respectively.

The power transmission is changed by changing the displacement of thehydraulic pump unit 10 a in a known manner, by pivoting the pump unit.

The power requirement and power transmission are changed, by changingthe displacement of the hydraulic motor unit 10 b in a known way, bypivoting said unit. The shaft 1 g and the shaft 8 are shown to rotate atthe same speed in the illustrated case. Means for changing thedisplacement and therewith the rotational speed of the output shaft areknown and will not therefore be described in detail here.

FIG. 3 illustrates the principle function of the planetary geararrangement “P” in the low speed range, wherein the rotary movement ofthe first shaft 1 a is transmitted to the hydraulic system 10 via thedrives 1 b, 1 d and 1 f and the gear 9, wherein said hydraulic systemsolely influences rotation of the second-shaft 8, with the aid of a pumpunit 10 a driven by said hydraulic system and a motor unit 10 b drivenby the pump unit 10 a. It follows from this that the planetary geararrangement 1 does not participate in the transmission of the speedratio between the shafts 1 a and 8, but functions solely as a drivesource for the hydraulic pump unit 10 a.

FIG. 4 is intended show that the function of the planetary geararrangement 1 is utilised, in that the ring gear 1 e is rotatablymounted so that the output shaft 1 g, 8 will be driven by the drive 2,and in that the speed at which the ring gear 1 e rotates will bedetermined by the extent to which the hydraulic system 10 is utilised.

In the high speed mode with the output shaft 8 rotating at a high speed,the motor unit 10 b and the gear 2 are driven at the same speed at ahigh r.p.m. The motor unit 10 b runs lightly with low displacement orwith no displacement, wherewith the pump unit 10 a runs sluggishly andslows down the gear 9 and the gear 1 f and the rotary movement of theplanet gears 1 d and the shaft 1 a is transmitted to the ring gear 1 e.

With diminishing speed under this setting, the pump unit 10 a may be setfor maximum displacement and the motor unit 10 b is angled out forgreater and greater displacement. This increased change in thedisplacement of the motor unit 10 b can continue until the pump unit 10a rotates at a maximum permitted speed.

This can be achieved with the aid of means for stopping rotation of themotor unit, so as to obtain a given displacement ratio.

This displacement ratio can also be implemented when dimensioning therespective displacement of the motor unit and the pump unit.

FIG. 5 is a perspective view of one-embodiment of a power transmissionsystem that includes a planetary gear arrangement “P” in accordance withthe invention and shows the shaft 1 a, the sun gear 1 b, the planetgears 1 d and the ring gear 1 e. The lines or conduits of the hydraulicsystem have not been shown in FIG. 5. As will be evident from FIG. 5,the toothed ring or gear if holding the planet gears co-acts with a gearor drive 9 which is coupled directly to the pump unit 10 a.

It will also be seen from FIG. 5 that the motor unit 10 b is coupleddirectly to the gear or drive 6.

FIG. 6 shows that the coupling unit 3 can be moved between an active andan inactive state or position with the aid of piston-cylinder devices 3a and 3 b while using hydraulic pressure in an hydraulic line 3 c.

Correspondingly, the coupling unit 5 includes piston-cylinder devices 5a, 5 b which are also activated between an active and inactive positionwith the aid of hydraulic pressure in an hydraulic line 5 c.

The conditions required to operate at a low speed within said low speedrange with the aid of a first embodiment of an hydraulic couplingarrangement will now be described in detail with reference to FIG. 7.

The brake unit or friction disc 3 is activated in this position, and thedrive 2 is therewith locked to the chassis 4 and the ring gear 1 e isconsequently stationary.

The planet gears 1 d will now be driven directly by the shaft 1 a andthe sun gear 1 e, wherewith the gear 1 f or the planet gear carrier isalso driven.

A gear 9 coupled directly to the gear 1 f now drives the hydraulic pump10 a, said pump being coupled to drive the hydraulic motor 10 b via twohydraulic lines 10 c, 10 c′.

The hydraulic motor 10 b is coupled directly to the gear 6, said gearbeing attached to the shaft 1 g and therewith drives the gear 7 and theoutput shaft 8.

The gear 6 co-acts with a coupling unit or a friction lining 5 adaptedto provide co-action between the rotary movement of respective gears 2and 6.

The gears 2 and 6 can rotate freely in relation to one another in aninactive position, and are adapted for common rotary movement in anactive position.

In this regard, it may be suitable to allow the motor 10 b to initiallytake a position for maximum displacement, wherewith the displacement ofthe pump unit 10 a driven by the shaft 1 a will increase such as toincrease the rotational speed of the motor unit 10 b and therewith theshaft 8.

In the state shown in FIG. 8, the pump-unit 10 a is pivoted or swung toa position for maximum displacement, while the motor unit 10 b stilltakes a position for maximum displacement.

In other respects, this Figure illustrates a second embodiment of anhydraulic coupling arrangement.

FIG. 9 shows the motor unit 10 b swung to a position in whichdisplacement is decreased, whereas the pump unit 10 a still takes aposition for maximum displacement, so as to further increase the speedat which the shaft 8 rotates.

FIG. 10 illustrates the settings of the hydraulic pump 10 a and thehydraulic motor 10 b at a low speed within the high speed range.

In order for the power transmission system to adopt a mode in the highspeed range, it is necessary for the coupling unit 3 to take an inactiveposition and to enable rotation of the gear 2 to be transmitted to theoutput shaft 8 (via the gear 6 and the shaft 1 g).

The motor unit 10 b is coupled directly to the shaft 1 g and since themotor unit 10 b is only slightly angled and set for a small displacementand coupled hydraulically with the pump unit 10 a, which is angled formaximum displacement, the gear or wheel 1 f will rotate rapidly sincethe pump unit 10 a exerts only a small force on the gear 9.

The gear 1 f or planet gear holder can now rotate at a speedcorresponding to the speed of the hydraulic motor shaft 1 g, this speedbeing governed by displacement differences between pump unit 1 a andmotor unit 10 b. The high rotational speed of the planet gears 1 d willlower the rotational speed of the ring gear 1 e.

At full speed within the high speed range, the motor unit 10 b accordingto FIG. 11 will be set to minimum displacement, whereas the pump unit 10a will be set to maximum displacement, whereby the gearwheels 1 f and 9will be stationary and the rotary movement of the shaft 1 a will passdirectly to the shaft 8 via the planet gears 1 d, the ring gear 1 e andthe gears 2, 6 and 7.

The coupling of the motor unit 10 b with the pump unit 10 a shown inFIG. 12 applies to the overdrive function.

An hydraulic system 10 that can be used in conjunction with the presentinvention will be apparent from FIGS. 7 and 13. These Figures show theuse of an hydraulic pump 10 d and a pair of valve arrangements 10 e, 10f, the methods of operation of which will be evident from the symbolsgiven in FIG. 13.

With regard to the valve arrangement 10 e, it will be apparent that thisarrangement takes one and the same position in FIGS. 7-11 and therewithenables the pump unit 10 a to be coupled directly with the motor unit 10b, whereas FIG. 12 shows that the valve unit 10 e switches the motorunit to the pump unit, and vice versa. A change in the direction ofrotation of the output shaft (reversing) takes place in this case.

It will also be seen that the valve unit 10 f drives hydraulic pressureon the line 5 c in FIGS. 7, 8 and 9, while hydraulic pressure occurs onthe line 3 c in FIGS. 10, 11 and 12.

It will be seen in particular from FIG. 13 that the hydraulic pump 10 dis coupled directly to the shaft 1 a and therewith driven at a constantrpm.

Check valves 10 h, 10 h′ have been connected between the lines 10 c, 10c′ and a pressure limiting facility is shown at 10 k.

The reference 10 m designates a “shuttle” valve and reference 10 pdesignates a pressure-reducing valve.

It will be noted that the hydraulic pump unit 10 a can be made variable,so as to be able to vary the pump unit within the positive range andalso via 0-ranges over to the negative range (back function).

It will also be noted that the transition from hydrostatic operation tosplit-torque operation can take place via an intermediate H-coupling invalves 10 e, meaning that the hydraulic actuation is disengaged(freewheel).

It will be understood that the invention is not restricted to theaforedescribed and illustrated exemplifying embodiments thereof and thatmodifications can be made within the scope of the inventive concept asdefined in the following Claims.

What is claimed is:
 1. A power transmission system that includes arotatable and drivable first shaft which is adapted for coaction with amotor that forms part of a planetary gear arrangement, and a secondshaft which is driven by said arrangement, wherein a planet gearbelonging to the planet gear arrangement includes a centrally positionedsun gear that coacts with said first shaft, a number of planet gearsable to coact with said sun gear, a ring gear coacting with said planetgears, and a gear holding said planet gears, and wherein an hydraulicsystem forming part of the planetary gear arrangement includes a pumpunit and a motor unit, among other things, and wherein the ring gearbelonging to the planetary gear mechanism can be braked via a firstcoupling unit and therewith held stationary relative to a chassis in alow speed range while being freed for rotation in a high speed range,wherein in the low speed range the hydraulic system functions to drivethe second shaft with the pump unit and the motor unit but provides asplit-torque function in the high speed range, characterized in that thefirst shaft is adapted to rotate the sun gear; in that said hydraulicpump unit is coupled directly or indirectly to the gear holding saidplanet gears; in that said hydraulic motor unit is coupled directly orindirectly to the second shaft; in that a second coupling unit functionsto co-ordinate the rotary movement of the ring gear with the pivotalmovements of the hydraulic motor in the high speed range; and in thatthe first coupling unit is adapted to act between a chassis and a gearco-acting with the ring gear; and in that the second coupling unit isadapted to act between a gear co-acting with said motor unit and thegear co-acting with said ring gear for common rotary movement androtation of the second shaft.
 2. A system according to claim 1,characterised in that said first coupling unit is in an active coupledposition in said low speed range while the second coupling unit is in aninactively coupled position; and in that said first coupling unit is inan inactively coupled position in said high speed range whereas thesecond coupling unit is in an actively coupled position.
 3. A systemaccording to claim 1, characterised in that in said low speed range saidfirst shaft functions to drive said pump unit via the gear that holds orcarries the planet gears and a gear connected to said planet gearcarrier.
 4. A system according to claim 1, characterised in that saidfirst and/or said second coupling unit can be activatable andde-activatable via a valve arrangement belonging to an hydraulic system.5. A power transmission system that includes a rotatable and drivablefirst shaft which is adapted for coaction with a motor that forms partof a planetary gear arrangement, and a second shaft which is driven bysaid arrangement, wherein a planet gear belonging to the planet geararrangement includes a centrally positioned sun gear that coacts withsaid first shaft, a number of planet gears able to coact with said sungear, a ring gear coacting with said planet gears, and a gear holdingsaid planet gears, and wherein an hydraulic system forming part of theplanetary gear arrangement includes a pump unit and a motor unit, amongother things, and wherein the ring gear belonging to the planetary gearmechanism can be braked via a first coupling unit and therewith heldstationary relative to a chassis in a low speed range while being freedfor rotation in a high speed range, wherein in the low speed range thehydraulic system functions to drive the second shaft with the pump unitand the motor unit but provides a split-torque function in the highspeed range, characterized in that the first shaft is adapted to rotatethe sun gear; in that said hydraulic pump unit is coupled directly orindirectly to the gear holding said planet gears; in that said hydraulicmotor unit is coupled directly or indirectly to the second shaft; inthat a second coupling unit functions to co-ordinate the rotary movementof the ring gear with the pivotal movements of the hydraulic motor inthe high speed range; and in that in said low speed range said firstshaft functions to drive said pump unit via the gear that holds orcarries the planet gears and a gear connected to said planet gearcarrier.
 6. A power transmission system that includes a rotatable anddrivable first shaft which is adapted for coaction with a motor thatforms part of a planetary gear arrangement, and a second shaft which isdriven by said arrangement, wherein a planet gear belonging to theplanet gear arrangement includes a centrally positioned sun gear thatcoacts with said first shaft, a number of planet gears able to coactwith said sun gear, a ring gear coacting with said planet gears, and agear holding said planet gears, and wherein an hydraulic system formingpart of the planetary gear arrangement includes a pump unit and a motorunit, among other things, and wherein the ring gear belonging to theplanetary gear mechanism can be braked via a first coupling unit andtherewith held stationary relative to a chassis in a low speed rangewhile being freed for rotation in a high speed range, wherein in the lowspeed range the hydraulic system functions to drive the second shaftwith the pump unit and the motor unit but provides a split-torquefunction in the high speed range, characterized in that the first shaftis adapted to rotate the sun gear; in that said hydraulic pump unit iscoupled directly or indirectly to the gear holding said planet gears; inthat said hydraulic motor unit is coupled directly or indirectly to thesecond shaft; in that a second coupling unit functions to co-ordinatethe rotary movement of the ring gear with the pivotal movements of thehydraulic motor in the high speed range; and in that the motor unitincluded in the hydraulic system is set to zero and freely rotatable ina position for the higher speed adjustment; in that the pump unit istherewith latched and stands stationary, wherein rotary movement of thefirst shaft is transferred to the second shaft via the sun gear, planetgears and ring gear.
 7. A power transmission system that includes arotatable and drivable first shaft which is adapted for coaction with amotor that forms part of a planetary gear arrangement, and a secondshaft which is driven by said arrangement, wherein a planet gearbelonging to the planet gear arrangement includes a centrally positionedsun gear that coacts with said first shaft, a number of planet gearsable to coact with said sun gear, a ring gear coacting with said planetgears, and a gear holding said planet gears, and wherein an hydraulicsystem forming part of the planetary gear arrangement includes a pumpunit and a motor unit, among other things, and wherein the ring gearbelonging to the planetary gear mechanism can be braked via a firstcoupling unit and therewith held stationary relative to a chassis in alow speed range while being freed for rotation in a high speed range,wherein in the low speed range the hydraulic system functions to drivethe second shaft with the pump unit and the motor unit but provides asplit-torque function in the high speed range, characterized in that thefirst shaft is adapted to rotate the sun gear; in that said hydraulicpump unit is coupled directly or indirectly to the gear holding saidplanet gears; in that said hydraulic motor unit is coupled directly orindirectly to the second shaft; in that a second coupling unit functionsto co-ordinate the rotary movement of the ring gear with the pivotalmovements of the hydraulic motor in the high speed range; and in thatsaid first and/or second coupling unit can be activatable andde-activatable via a valve arrangement belonging to an hydraulic system.